Known electrical equipment includes field devices used, for example, as measured-value sensors for pressure, temperature, flow rate and the like, as actuators for the control and operation of actuating elements such as valves or flaps, or as communication components for data interchange between interoperating automation-technological apparatuses.
Special safety regulations are applicable to the use of electrical equipment in surroundings subject to explosion hazards in process engineering installations.
For example, safety regulations may stipulate that in the event of a short circuit of the cores of the supplying conductor loop, no energy that may lead to the ignition of the surrounding atmosphere subject to explosion hazards is permitted to be fed back from the field device into the conductor loop. It is therefore desirable to permit the current flow toward the field device and to block the inverse current flow from the field device. It is generally familiar to the person skilled in the art, without separate proof being necessary, that diodes have a forward direction, in which the current flow through the diode is possible, and a reverse direction, in which the current flow through the diode is blocked, and consequently, when inserted into the supply line, diodes can satisfy the desire for feedback protection.
Furthermore, a redundant design of safety-relevant components is desirable for safety reasons. This gives rise to a series circuit composed of, for example, at least two, or owing to particular national regulations three, diodes. Such a known protective circuit is shown in FIG. 1. Proceeding from a voltage drop per diode in the forward direction of approximately 0.6 V at room temperature, a total voltage drop across the protective circuit according to FIG. 1 amounting to 1.8 V at room temperature results, which, as the ambient temperature decreases, rises up to 3.0 V at −40° C. In the case of a field device arrangement supplied via a current loop, the voltage drop across the protective circuit can lead to an undesirable reduction of the range of the current loop.